Sure, sometimes I've had cleanup/integrity/whatever tasks that run every few minutes, but the amount of records affected has always been pretty small.
That's an interesting conundrum. See, this is why we're all messing around with Cassandra et al; sometimes, in SQL, the answer is "don't do that", because it'd be too hard to tailor the db's behavior to suit your needs. Although frankly, with a design that deletes and updates a significant percentage of records in the system on a certain schedule, I can see any number of storage solutions having trouble.
That's interesting. I'll be thinking about this at "work" today. ;) I have a bunch of comments, but they're of the half-baked "oh, what about this?" variety.